1. Field of the Invention
This invention relates to a shift control method for an automatic transmission of an automotive vehicle, and more particularly, to a shift control method capable of optimally carrying out a shifting of the automatic transmission from a neutral position to a reverse or drive position when the vehicle is moved from standstill.
2. Description of the Related Art
Automatic transmissions for vehicles have an input shaft to which the rotating force of an engine is input, an output shaft for outputting a driving force to a drive wheel side, a shift gear mechanism provided between the input and output shafts, and engaging elements utilizing friction, such as clutches, brakes, etc. The engaging elements are selectively operated to provide a predetermined speed ratio by means of the shift gear mechanism. These engaging elements are actuated by a fluid pressure, more particularly, by operating oil supplied thereto in accordance with a shift command from a selector lever of the vehicle or a shift command based on the operating conditions of the vehicle. Thus, rotating elements in the shift gear mechanism are appropriately selected, whereby a desired shifting is carried out manually or automatically.
In the automatic transmission described above, when the selector lever is shifted from the neutral position to the reverse position, a front clutch and a low/reverse brake (L/R brake) are selected from among a plurality of engaging elements and operated. Moreover, when the L/R brake is operated, the pressure of the operating oil supplied to the L/R brake, i.e., the L/R supply pressure, is controlled in accordance with a preset control pattern. By doing this, the response characteristic of the shifting of the automatic transmission from the neutral position to the reverse position is improved and shifting shock is reduced. Specifically, the L/R supply pressure for the L/R brake is controlled by a pressure control solenoid valve, which is subjected to duty control, and a neutral/reverse control valve.
The above-mentioned control pattern generally comprises a precontrol pattern executed in a preliminary stage from the input of a shift command until play in the mechanism of the engaging element, i.e., the L/R brake, is eliminated, and a main control pattern for actually operating the L/R brake in an engagement stage subsequent to the precontrol pattern. In these control patterns, the duty factor of the pressure control solenoid valve is defined appropriately in relation to the lapse of time.
Nevertheless, since the control patterns for controlling the L/R supply pressure of the L/R brake are not prepared for each of automatic transmissions, but are used in common, the relationship between the duty factor of the pressure control valve and the L/R supply pressure differs from one automatic transmission to another. Accordingly, when the L/R supply pressure is controlled in accordance with the control patterns, play in the mechanism of the L/R brake may not be eliminated even after the precontrol pattern is finished, or conversely, the L/R brake may be operated actually before the completion of the precontrol pattern. Thus, when the L/R supply pressure is thereafter controlled in accordance with the main control pattern, a variation occurs in the shift start time between the start of the main control pattern and the start of an actual shifting of the automatic transmission, making it difficult to stabilize the response characteristic of shifting.
Furthermore, since the duty factor of the pressure control valve at the start of controlling the L/R supply pressure in accordance with the main control pattern, i.e., at an initial stage of engagement, is set at a fixed value, the value of the L/R supply pressure at the initial stage of engagement also differs from one automatic transmission to another. Therefore, if the L/R supply pressure is too high at the initial stage of engagement, a peak occurs in the torque variation of the output shaft during the period between the start of a shifting of the automatic transmission and the end of same, and thus, the feeling at shifting is lowered. Conversely, if the L/R supply pressure at the initial stage of engagement is too low, the shifting of the automatic transmission is delayed.